THE EFFECT OF ALLOYING ELEMENTS ON THE FIRESIDE CORROSION OF 40 Ni ALLOYS IN SIMULATED WASTE TO ENERGY ENVIRONMENTS

摘要

Laboratory corrosion tests were conducted to clarify the effects of addition of the alloying elements silicon and aluminum on the fireside corrosion of austenitic 20wt.% Cr -40% Ni alloy in simulated waste to energy (WTE) plants. Test results were compared with those of Mo -added alloys obtained from the literature. Laboratory -melted alloys of up to 2.9% Si and up to 4.2% Al were prepared and coupon specimens of these alloys were reacted with synthetic ashes of a predominantly chloride salt mixture of 38.3wt.% Cl and of a sulfate -chloride salt mixture of 19.2% Cl -19.4% S (as SO3) in a gaseous environment of 1500 ppm HCl -300ppm SO2 -7.5% O2 - 7.5% CO2 -20% H2O -bal.N2 (by volume) at 400-550°C for 20 h. Addition of Al to the alloys increased the corrosion resistance to chloride salt at 500°C, and to the sulfate/chloride salt mixture at 550°C, but the effect of Al on corrosion was generally small and limited. For the chloride salt at 500°C, a drastic reduction of the corrosion weight loss was noticed for Si -added alloys, but the beneficial effect of Si was not prominent for the sulfate/chloride salt mixture at 500°C and 550°C. For waterwall tubes where chloride salts deposit, materials forming uniform Al2O3 and/or SiO2 scales might be effective. On the other hand, for superheater application where chloride/sulfate salt mixtures will deposit on tube surfaces, selecting high -Si and/or high -Al alloys may not necessarily be an effective measure to reduce the corrosion rate of tube alloys. For these corrosion environments, using alloys of high-Mo content such as alloy 625, in addition to good furnace design and careful operation, is considered the right direction to combat the corrosion in actual WTE plants.